During operation, turbine blades, particularly the last stage turbine blades in a steam turbine, experience water droplet erosion due to their location in the steam path. Over time, this erosion produces a jagged edge along the leading edge of the turbine blade. The jagged edge, in turn, acts as a notch, producing local stress concentrations in the eroded leading edge. As the erosion increases, the notches grow deeper in and become more pronounced, i.e., the differential between the low and high spots of the jagged edge become greater. This results in increased stress concentrations and, ultimately, may cause a failure in the turbine blade. In extreme cases, if the correct loading and eroding conditions coexist, the entire tip of the turbine blade may break free, which will cause significant damage to the turbine.
As a result, there is a need for an efficient and reliable manner to track and quantify the erosion of steam turbine blades, especially those in the last stage of a steam turbine. In general, the erosion of a set of turbine blades (i.e., the turbine blades within single stage of the turbine) may be tracked by periodically recording the chord length measurements of each of the turbine blades within a turbine blade set. As used herein and as one of ordinary skill in the art will appreciate the chord length of a turbine blade generally may be defined as the width of the airfoil of the turbine blade. More specifically, but not by way of limitation, chord length may be defined as the linear distance from the leading to the trailing edge on the suction side of the airfoil of the turbine blade. From the chord length measurements, the erosion may quantified and, based on the quantification, decisions may be made regarding when replacement of a turbine blade or a set of turbine blades is needed.
However, the constraints related to taking the necessary measurements of the turbine blade chord length are considerable. First, in general, downtime for turbine power generators is limited, which means there is limited opportunity to take the necessary measurements. As a result, the necessary chord length measurements must be able to be taken during a relatively short time period. This issue is exacerbated by the number of turbine blade measurements required to accurately ascertain the erosion level of a set of turbine blades. A turbine has multiple stages, and each turbine stage often contains between 80-100 turbine blades. Further, multiple measurements (approximately 10-20 separate measurements) are required along the length of each turbine blade to accurately quantify and describe its overall erosion level.
Second, because disassembling the turbine would be too time consuming and costly, the measurements must be taken with the turbine in the assembled condition, i.e., with the turbine blades assembled on the rotor within the turbine. Access to the assembled turbine blades to take the necessary measurements is difficult and limited. Any device used to take the necessary measurements must be able to fit between the turbine blades in their assembled condition. Further, in general, access to the assembled blades only occurs from an opening that allows a person to enter the turbine unit from a downstream location. As such, the person taking the measurements has direct access only to the downstream faces of the turbine blades. Thus, any device used to take the necessary measurements must be portable so that an operator may carry it into the turbine and be able to be used by an operator who has access to the assembled turbine blades from only a downstream location.
In general, chord length measurements are taken often (approximately every 6 to 18 months) so that information regarding the erosion level is as up-to-date and accurate as possible. However, current processes for obtaining the measurements are lengthy, inefficient and result in wasted man-hours and increased downtime to the turbine unit. Further, current devices and methods do not yield accurate or reliable data. Therefore, there is a need for a device that provides accurate and reliable turbine blade measurements in a time efficient manner.